Long-day Onion Research Articles

Yield and traits of bulb quality in the Spanish sweet onion cultivar ‘Fuentes de Ebro’ after selection for low pungency

The cultivar ‘Fuentes de Ebro’ is a mild, long-day onion grown in the Northeast of Spain. In this work we report the achievements of two cycles of selection in order to produce an ideotype that combines high yield and desirable bulb quality characters such as low pungency, high soluble solid content, adequate firmness and medium size. We first evaluated 12 selected half-sib families and the phenotypic variation observed for these four traits was analyzed, showing that these bulb characters were significantly affected by location, family and the interaction of both factors. The combined selection produced a significant decrease on pungency, while the other bulb quality related traits showed values similar to those of the unselected progeny. As pungency is positively correlated to bulb soluble solid content and negatively correlated to weight, our selection for multiple traits probably limited the unfavorable consequences of producing milder onions, which usually show large sizes, and low firmness and soluble solid content that reduces bulb conservation. We could estimate the realized heritability of pungency as H=0.75, which is in agreement to previous studies. The two families with significantly lower pungency levels were reproduced and their progenies evaluated again. One of these families that showed lower pungency levels than unselected and commercial plants, while maintained soluble solid content, firmness and weight traits, is going to be submitted to be officially recognized as a conservation variety, which may guarantee the integrity of the desirable attributes that reside in the ‘Fuentes de Ebro’ cultivar.

(49) Effects of Sowing Time and Tray Size for Production of Onion

Onions are a major vegetables in Korea. Short-day onions are grown in more than 95% in southern area of South Korea, and long-day onions are grown above 600-m altitude in the highlands. Onion cultivation in the highland areas has become difficult for farmers to obtain high income but stable production because of higher cost of seed and the intensive labor involved in production. Consequently, onion set culture by inexpensive onion seeds compared with expensive F1 seed has an advantage due to the higher proportion of marketable size bulbs. This study was conducted to find out the suitable sowing time and tray size for onion set culture in the highlands—the earlier the sowing time, the higher the number of onions set. The most appropriate sowing date and 1.6–3.5 g of onion set size was early to middle May, also with the highest distribution percentage. The highest set number (780.8 set/m2) of suitable onion set size (1.6–3.5 g) were harvested from the 288 trays, followed by 770 set from the bed (9 g/m2 of seed sowed), and (7/m2 of seed sowed) in the 406 tray.

Onion Seed Quality

High seed quality is required to obtain optimal plant stands and to achieve maximum yield potential. Onion seeds are commonly coated and treated with an insecticide and fungicide to control early season insects and diseases. The seed treatments may cause phytotoxicity in standard germination tests, and thus reduce the percent germination of a seed lot. The objective of this study was to examine the effect of seed quality on seed storage, stand establishment and yield. Two seed lots of two long-day onion varieties were pelleted and treated with the seed treatments Trigard and Pro-Gro. Standard germination and saturated salt aging tests were performed on each seed lot, indicating a high and lower quality seed lot for each variety. Aging was conducted by equilibrating pelleted seeds at 35% relative humidity and aging at 25 °C for up to 12 months. Aging decreased germination with time, especially for the lower quality lots. Field studies were conducted in two locations in upstate New York. The plant stand was greater in the high compared to the lower quality lots in both varieties at both locations. Yield was reduced in the lower quality than high quality lot; however, the response differed by variety and location. Overall, the initial seed quality had an impact on seed storagability, stand establishment and yield.

Plant Population and Nitrogen Fertilization for Subsurface Drip-irrigated Onion

Onion (Allium cepa L.) production in the Treasure Valley of eastern Oregon and southwestern Idaho has been based on furrow irrigation with 318 kg·ha-1 N fertilizer and average yields of 70 Mg·ha-1, but these practices have been implicated in nitrate contamination of groundwater. Drip irrigation, introduced in the early 1990s, has several advantages, including reduced leaching losses. Since onion plant populations and N fertilizer rates can affect economic returns, studies were conducted in 1999, 2000, and 2001 to determine optimum plant populations and N fertilizer rates for subsurface drip-irrigated onion. Long-day onion (`Vision') was subjected to a combination of seven nitrogen fertilization rates (0 to 336 kg·ha-1 in 56-kg increments applied between late May and early July) and four plant populations (185, 250, 300, and 370 thousand plants/ha). Onion was grown on silt loam in two double rows spaced 0.56 m apart on 1.1 m beds with a drip tape buried 13 cm deep in the bed center. Soil water potential was maintained nearly constant at -20 kPa by automated irrigations based on soil water potential measurements at a 0.2-m depth. Onion bulbs were evaluated for yield and grade after 70 days of storage. Onion yield and grade were highly responsive to plant population. Onion marketable yield increased, and bulb diameter decreased with increasing plant population. Within the range of plant populations tested, gross returns were not always responsive to plant population. Returns were increased by the increase in marketable yield obtained with higher plant population, but higher plant population also reduced the production of the largest sized bulbs which had the highest value per weight. Onion yielded 95 Mg·ha-1 with no applied N fertilizer, averaged over plant populations and years. Onion yield and grade were not responsive to N fertilizer rate or interaction of N fertilizer rate with plant population. Preplant soil available N, N mineralization, and N in irrigation water all contributed N to the crop. Onion N uptake did not increase with increasing N fertilizer rate.

Effects of water stress at specific growth stages on onion bulb yield and quality

One onion ( Allium cepa L .) quality parameter, the percentage of single-center bulbs, has become important to meet demands of both processing and fresh market buyers, who generally require a minimum of 70% single-center bulbs. Field studies on commercial drip-irrigated farms were conducted in 1999, 2000, and 2002 to determine the effects of soil-water stress imposed at different plant growth stages on long-day onion yield and quality including the percentage of single-center bulbs. Irrigation was withheld at four onion growth stages (3, 5, 7, and 9-leaf) or not at all in the control. In addition, one treatment had water stress imposed at both the 3 and 7-leaf stages. When soil-matric potential was ≤−70 kPa (approximately 40–50% depletion of available water for the soils at the sites) at 0.15 m soil depth, irrigation was restored to farmer’s normal levels for the duration of the growing season. Analysis of the three-year data, showed that total yield was reduced by soil-water stress imposed at any growth stage but the greatest effect was at the 5-leaf, 7-leaf, and 3- and 7-leaf stages. Soil-water stress caused by withholding irrigation at both the 3- and 7-leaf stages reduced yields by 26% compared with the control. The percentage of single-center onion bulbs was lower when the soil-water stress occurred earlier in the growing season than when the stress occurred later. Based on the three-year data analysis, only the control and 9-leaf treatments met the industry standards for percent single-center bulbs. Compared to the control, the percentage of single-center bulbs was reduced by 40, 32, and 18% when soil-water stress was imposed at the 3- and 7-leaf, 3-leaf, and 5-leaf stages, respectively.

Evaluation of Long-day Onions for Resistance to White Rot Infection Using Greenhouse and Laboratory Techniques

Breeders have found field screening for onion white rot resistance to be unreliable since consistently moderate to high disease levels that significantly differentiate cultivars, do not occur over field sites and years. The objective was to see if a greenhouse or laboratory technique could predict field reaction of onion accessions. Onion (Allium cepa) accessions were grown in fields naturally infested with the white rot causing fungus (Sclerotium cepivorum) in 1999 and 2000 (New Zealand) and in 2000 and 2001 (Canada). The field disease levels were low at three sites, moderate at two and high at one. Field screening was not a reliable predictor of white rot reaction when disease incidence was low. Onion accessions were screened for resistance in the greenhouse using nonsterile muck soil (NSMS) and sterile muck soil (SMS) with S. cepivorum sclerotia as the inoculum source. Total disease incidence was significantly higher in the NSMS compared to the SMS and accessions showed significant variability for white rot reaction in both soils. Two laboratory-based techniques were used to test the effect of onion volatiles on mycelium growth in culture. The volatiles from susceptible accessions resulted in faster radial growth of S. cepivorum mycelium (on water agar) and height of aerial mycelium (on potato dextrose agar) than volatiles from resistant accessions. Disease incidence in the greenhouse, S. cepivorum culture growth rates on water agar media and aerial mycelial height were all good predictors of field disease incidence in a covariance analysis. The best predictor was aerial mycelial height, which was predictive of field disease incidence in four out of six field sites. Onion breeders can use the methods described in this study in breeding for white rot resistance.

Comparison of direct-seeded, bareroot, and various tray seedling densities on growth dynamics and yield of long-day onion

Comparaison entre le semis direct, la transplantation a racine nue et diverses densites de plants en godets dans la dynamique du developpement et la production d'oignon de jour long. Developpement d'oignon transplante a densite augmentee. Le developpement de la racine, du bulbe et du tige d'oignon transplante obtenu a partir de plants en godets de 228 compartiments a ete etudie. Les traitements ont ete un (TC1), deux (TC2), ou trois (TC3) plants/compartiment, et une plante separee dans la transplantation de celles developpees avec deux (TC2-1) ou trois (TC3-1) plants/compartiment. Les controles ont ete: semis direct (DC) et transplantation a racine nue (BR). Les plantes transplantees ont ete disposees en rangee avec les distances suivantes: 5 cm (BR, TC1, TC2-1, TC3-1), 10 cm (TC2), et 15 cm (TC3). Pendant le premier et le mi-developpement le nombre de racines et le poids sec de racines a ete superieur pour TC 1 compare avec BR. La masse de feuilles des plantes transplantees s'est dessechee dans les 64 et les 80 jours apres la transplantation (DAT). Initialement, DS a eu un taux de formation de bulbe rapide, tandis que les plantes transplantees ont montre un taux plus rapide 60 DAT. Avant la maturite finale, TC1 a eu une taille des bulbes plus grande que DS et TC3. Les productions totales ont ete semblables, mais la production en classes de taille a differe de maniere significative entre les traitements. DS et TC3 ont eu une augmentation dans la taille des bulbes petits (35-50 mm de diametre). Tous les plants en godet, excepte TC3, ont eu une augmentation significative dans la taille des bulbes grands (60-70 mm). TC1 a eu 36 % de la production totale commercialisable avec la taille jumbo (> 70 mm) par rapport a 16 % pour le semis direct et a 24 % pour la transplantation a racine nue.

Changes in non-structural carbohydrate composition during bulbing in sweet and high-solid onions in field experiments

The composition of non-structural carbohydrate (NSC) content of predominantly long-day onion germplasm has been assessed over several years and in a bi-location trial. It was observed that genetic rather than environmental factors determined the NSC composition of onion bulbs. Glucose was the NSC component which was most closely correlated with genotype. Fructose was the only NSC component that was significantly affected by environment. Sucrose and 1-kestose (DP3 fructans) were not correlated to a large extent to the other NSC components, indicating their transient role in the fructan metabolism. Strong negative correlations were observed between reducing sugars (i.e. fructose and glucose) and dry matter content (DM). Furthermore, it was shown that accessions differed significantly in their fructan accumulation pattern; high DM accessions showed accumulation of fructans over the whole bulbing period, whereas, low DM accessions quickly reached a plateau. Implications for the breeding of high quality onions are discussed.

Non-structural carbohydrate composition of long-day onion in vitro

SummaryBulb formation in onion (Allium cepa L.) is strongly influenced by environmental factors, particularlyby light via photosynthesis and daylength. Dry-matter content (DM) in an onion bulb varies between 6 and 25%. Of this, 60–80% is made up of non-structural carbohydrates (NSC). This study compared the relative influence of genetics and environment on the concentration and composition in NSC of onion bulbs. The effects of clonal genotype, light spectrum quality and sucrose concentration in the culture medium were investigated in vitro on a long-day onion type. NSC analyses were performed enzymatically and by HPLC on growing material. Fructans characterized the basal tissues, and the sucrose content in the medium influenced the NSC distribution of the onion tissues. Bulb induction by supplementary incandescent light stimulated fructan accumulation in basal tissues. The high DM clones stored fructans faster and at higher concentrations than the low DM ones. They expressed a higher potential for fructan sink or biosynthesis. A similar biochemical composition in vitro as field-grown bulbs resulted from the light spectrum quality of light.

Irrigation Criteria for Drip-irrigated Onions

Long-day onion (Allium cepa L. `Vision') was subjected to five soil water potential (SWP) treatments (–10, –20, –30, –50, and –70 kPa) using subsurface drip irrigation in 1997 and 1998. Onions were grown on 1.1-m beds with two double rows spaced 0.56 m apart and a drip tape buried 13 cm deep in the bed center. Soil water potential was maintained at the five levels by automated, high-frequency irrigations based on SWP measurements at 0.2-m depth. Onions were evaluated for yield and grade after 70 days of storage. In 1997, total and colossal (bulb diameter ≥102 mm) yield increased with increasing SWP, but marketable yield was highest at a calculated –21 kPa because of greater decomposition in storage in wetter treatments. In 1998 total, marketable, and colossal-grade onion yield increased with increasing SWP. Onion profits were highest with a calculated SWP of –17 kPa in 1997, and at the wettest level tested in 1998. Storage decomposition was not affected by SWP in 1998. Maintenance of SWP at –10 and –20 kPa required, respectively, 912 and 691 mm of water in 1997 and 935 and 589 mm of water in 1998. Onion crop evapotranspiration from emergence to the last irrigation totaled 681 mm in 1997 and 716 mm in 1998.

Onion Storage Decomposition Unaffected by Late-season Irrigation Reduction

Long-day onion (Allium cepa L.) `Vision' was submitted to four soil water potential (SWP) treatments using subsurface drip irrigation in 1997 and 1998. Onions were grown on two double rows spaced 22 inches (56 cm) apart on 44-inch (112-cm) beds with a drip tape buried 5 inches (13 cm) deep in the bed center. SWP was maintained at four levels by automated, high frequency irrigations based on SWP measurements at an 8-inch (20-cm) depth. The check treatment had SWP maintained at -20 cbar (kPa) during the entire season. The other three treatments had SWP maintained at -20 cbar until 15 July, then reduced to -30, -50, or -70 cbar. Reducing the SWP level after 15 July below -20 cbar failed to reduce onion bulb decomposition in storage, but reduced colossal onion yield in 1997, and marketable and total yield in 1998.

Postharvest Flux and Genotype × Environment Effects for Onion-induced Antiplatelet Activity, Pungency, and Soluble Solids in Long-day Onion during Postharvest Cold Storage

Two mild and two pungent onion (Allium cepa L.) selections (hereafter referred to as cultitypes), W420B, W424B, MSU8155B, and Exhibition, were grown at two locations in two states (Wisconsin and Oregon) during 1994 and 1995. Onion bulbs were harvested, stored at 4 °C and sampled for antiplatelet activity, pungency, and soluble solids 10 days after harvest and every 40 days during a 210-day postharvest storage period. Significant cultitype × state and cultitype × year interactions were detected. However, these were primarily due to the change in rank of cultitypes within the mild or pungent group. Averaged over all environments, antiplatelet activity was significantly greater in 1994 compared to 1995 for all cultitypes. Significantly greater antiplatelet activity was measured for three out of four cultitypes grown in Oregon compared to Wisconsin. During postharvest storage, antiplatelet activity increased 61% and 56% across all cultitypes and across both states during 1994, and across all cultitypes in Wisconsin during 1995, respectively. Although pungency determination can be a good indicator for relative rankings of different cultitypes for antiplatelet activity, changes in pungency were not correlated with changes in antiplatelet activity during postharvest storage. Results demonstrate cultitype, environment, duration of postharvest storage and genotype × environment interactions influence pungency, soluble solids, and antiplatelet activity, which should be considered when assessing onion-induced antiplatelet activity.

Variations in gynogenic response among long-day onion ( Allium cepa L.) accessions

Long-day onion genotypes originating from Europe, North America and Japan were tested for gynogenic induction ability. Haploid induction using an efficient single-step flower culture induction procedure was at least as productive as the previously studied double-step flower/ovary culture, while undesired callusing of flower bases did not substantially lower the induction frequency. Differences in embryo yields among different accessions and among different donor plants within accessions were very pronounced. Three extremely responsive accessions were found, with average embryo yields of 18.6–22.6%, while individual donor plants produced up to 51.7% embryos. In terms of geographic origin, genetic material bred in America was on average almost five and nine times more responsive than European and Japanese material, respectively. The stability of high induction frequency was confirmed by culturing flowers from the same individual donor plants in two subsequent growth years. Of the regenerants, 90.5% were haploid and 88.2% of analysed diploid regenerants were homozygous. The results indicate that the genotype of donor plants has a crucial influence on haploid induction ability and that the less labour intensive single-step flower induction procedure is an efficient method of obtaining a high-frequency homozygous-embryo induction rate.

Evaluation of Nitrogen Sources and Rates on the Yield and Quality of Sweet Onion

During the 1980s, research was conducted mainly for long-day onion, with limited research on cultural practices for short-day cultivars, in Uruguay. Sweet onion production became important in Uruguay in 1992. Research was conducted on cultivar adaptation, sowing time, plant population, transplanting and direct-seeded methods, irrigation, and plant protection. Onion fertilization criteria were established for long-day cultivars in the southern region in the 1980s. National Research Inst. of Agriculture started a study on sweet onion fertilization using four N rates, 0, 60 120, and 180 kg/ha, and four N sources, urea, ammonium nitrate, potassium nitrate, and calcium nitrate. Nitrogen was applied into the soil 40% before transplanting and the remaining 60% in two applications in the spring. Granex 33 cultivar was transplanted on 27 June 1997 into a soil with the following characteristics, pH 6.4, P (Bray 1) 44.5 ppm, organic matter 3.3, and K 0.64 meq/100 g. Beds were separated 1.5 t, with four rows and plants 12 cm apart. Plant height and leaf color were evaluated. Lack of nitrogen and 60 kg N/ha showed lighter green colored leaves. Leaf, total plant, and soil samples were collected to determine nutrient status and dry matter production. Bulbing ratio was measured in 10 plants in each plot. Sweet onion quality was done by pyruvic acid analysis.

Ploidy stability and in vitro chromosome doubling in gynogenic clones of onion ( Allium cepa L.)

Ploidy stability in gynogenic clones of long-day onion is assessed during micropropagation. In order to induce chromosome doubling in haploids, explants and plantlets of three clones were treated in vitro. Colchicine (0.625–12.5 mM) and oryzalin (10–200 μM) were tested for 24 h in combination with culture conditions (4°C in the dark or 24°C in the 16 h day photoperiod). Diploidization performed well (64%) during micropropagation on explants treated with 0.625 mM colchicine at 24°C day. Among treatments on individual plantlets, best results were obtained either with 2.5 mM colchicine (up to 65.7% of diploids) or 50 μM oryzalin (up to 57.1% of diploids). Both chemicals induced mixoploids and affected plant regeneration, but a better plant quality was obtained with oryzalin. Chromosome doubling in explants during the first cycle of micropropagation after gynogenesis appears to be a valuable and time-saving procedure to provide numerous doubled-haploid plants.

Environment and Storage Time Influence on Pungency and Sugar Profiles in Long-day Onions

Sweet onions have gained global importance because of their mild flavor and tear-free properties. A field investigation with seventeen Walla Walla Sweet onion entries grown at the IAREC of Washington State Univ. were evaluated in 1994 and 1995. The flavor and quality characteristics such as pyruvic acid and total sugar (fructose, glucose, and sucrose) concentrations were analyzed by spectroscopy and HPLC respectively. The pyruvic acid concentration in bulbs stored (5 °C and 65% to 75 % relative humidity) for 0, 2, and 4 months increased as the storage time increased. During 1994 and 1995 the pyruvic acid ranges during storage were 4.8 to 7.9 and 2.9 to 9.6 mmols·g–1, respectively. Total sugar concentration in 1994 decreased as the storage period increased, while in 1995 the trend was reversed. There was a higher concentration of sucrose in 1995. In general, Walla Walla onions showed a higher sugar: pyruvic acid ratio among different entries compared to short-day (Vidalia and Texas Grano 1015Y) onions.

Bulbing in Long-day Onion (Allium cepa L.) Cultured In Vitro: Comparison Between Sugar Feeding and Light Induction

Bulbs were obtained on onion plants cultured in vitro. No bulbing happened under long days with fluorescent light and 30–40 g l−1 sucrose. Bulb formation was mainly dependent either on sucrose concentration in the culture medium, or on light spectral composition. Raising the sucrose concentration from 40 to 120 g l−1 increased plant basal swelling and stopped further vegetative development. These plants were not dormant. When fluorescent light was enriched in incandescence during a long day period, bulbs were obtained in two months and underwent a consecutive dormancy. (Resume d'auteur)

Influence of Growth Regulators on Growth, Bulbing, Maturity and Yield in Onions1

Abstract Short, medium, and long-day onions (Allium cepa L.) were field grown under normal spring conditions of increasing daylength. Three growth regulators, (2-chloroethyl)phosphonic acid (ethephon), gibberellic acid (GA3), and succinic acid-2,2-dimethylhydrazide (SADH) were applied as sprays at different concentrations at the 4, 6, and 10 leaf stages of development. Ethephon reduced plant height and number of leaves and initiated earlier bulbing. Ethephon-treated plants also had a higher bulbing index (bulb/neck diameter) early in the season due to more rapid bulb enlargement but difference did not persist to maturity and at harvest ethephon significantly reduced both bulb diameter and yield. Ethephon failed to bring about earlier maturity based on plant lodging at harvest. GA3 increased the number of leaves per plant and percentage of plants initiating multiple growing points. No measurable responses to SADH were noted.

Effect of Ethephon on Bulbing of Onion under Noninductive Photoperiod1

Abstract (2chloroethyl)phosphonic acid (ethephon) induced bulbing of short-day, intermediate and long-day onion cultivars under noninductive short-day conditions. Seedlings with roots and sheaths constantly immersed in ethephon solution as low as 1.5 ppm initiated bulbing after 7 days. Application of ethephon as leaf spray, as soil drench, or by injection into the root growing zone, caused bulbing at minimum concentrations of 1200, 480 and 600 ppm, respectively. The number of leaves per plant was reduced in all treatments that caused bulbing, but the reduction was not significant in the soil drench 480 ppm treatment. Injection of ethephon solutions of 37.5 or 75 ppm to the root growing zone appeared to promote leaf emergence and did not initiate bulbing.